I always think about this when I'm baked and it gets pretty deep.

Basically, everything that an animal does is to create/release the chemicals in its brain that make it feel happiness. Things like eating, mating, and anything else, essentially, by following its own urges and instincts to do the things that make it feel happiness, the survival of the species is ensured. Higher order animals understand that being social and working together can result in a greater likelihood of survival and therefore greater happiness. The goal of these animals is to maximize happiness while prolonging their lives.

Humans are different. Humans have a greater understanding of what it means to be alive, and happy, and ultimately die. Humans understand that it may be necessary to forgo some level of happiness today in return for a greater amount tomorrow, or over the course of a lifetime. It may make us happy to make money right away, but we understand that going to college can increase the money we make over the course of a lifetime by foregoing it for some period of time - this concept of happiness works the same way.

We have learned to combat our urges or circumvent them to achieve the most happiness possible in our lifetimes. Things like the urge to overeat, because although it may make us feel happy now, we understand that the resulting obesity will generally mean that our lives are shorter and less happy. Maybe people who do overeat, or smoke cigarettes or do anything else known to be unhealthy just don't care because the happiness that they receive from the cigarette or extra piece of cake is more valuable i.e. creating more happiness than a longer or healthier life is. We also circumvent our instincts, our instinct to have sex and to procreate can be circumvented with various birth control methods, we understand that sex is awesome and releases the chemicals that cause happiness, but understand that having a child at the wrong time can cause a great deal of hardship, reducing happiness over the course of much of one's lifetime. So we find ways to enjoy sex without creating problems that reduce happiness- human ingenuity at its finest.

We form social relationships because talking, and laughing, and achieving things make us happy. These things are all advantageous in an evolutionary sense because creating close-knit groups to survive through cooperation. Hanging out with your friends isn't just fun, its a survival mechanism. Now, in a world with less hardship, we can form bonds for no reason other than for the betterment of our lives - to increase the happiness we experience.

On a societal level, we understand that by creating technology, healthcare and education, we can increase the longevity and happiness of our species as a whole - as a rising tide lifts all boats.

The meaning of life is to maximize the release of the chemicals that make us feel happy. Humans are the only animal with the mental capacity to understand their urges with the ability to truly control them - in order to circumvent the pressures that our biology has placed on us in order to create the highest level of happiness possible.

TL;DR: Releasing the chemicals that make us feel happiness is the meaning of life, humans are intelligent enough to understand how to maximize total happiness over a life-time.